1. Field of the Invention
The present invention relates generally to the field of microwave filters and more particularly to a bandpass filter which is to be used in microwave communication systems, such as cellular phones, cellular phone base stations, satellites and the like.
2. Description of the Related Art
In the microwave communications market, the microwave frequency spectrum has become severely crowded and has been subdivided into a vast number of different frequency bands. There is a need to design microwave filters that have an output signal only at a precise (narrow) frequency band. Also, it is necessary that this filter can be tuned to a precise frequency band with there being a separate filter for each precise frequency band.
In the field of microwave bandpass filters, it is known that the frequency band of the signal of the filter is a function of the resonant frequency of resonators that are incorporated within the filter and respective coupling coefficients between each of these resonators. Typically, in order to achieve a specific precise bandwidth, the resonators are longitudinally spaced in a sequential manner. The bandwidth is a function of the coupling between the resonators and the frequency of the resonance of the resonators. Varying of the spacing between the resonators results in variations in the bandwidth. Accordingly, overall filter dimensions, such as the filter length, typically must be varied in order to tune a filter to a precise bandwidth. Therefore, in the past in order to divide a microwave communications band into the many different frequency bands of operation, a multitude of different filter dimensions are necessary. However, because there is a need to minimize the size of such filters, and the fact that such filters may be located in very remote locations, such as satellites, a non-uniform filter dimension is just not acceptable.
The constructing of a filter that can be tuned to a selected microwave frequency has long been known. It has been discovered that if there is included in the filter a cross-coupler that connects between a pair of non-sequential resonators, a variation in the response of the filter is obtained. A slight position variation of that cross-coupler will result in a mismatch of the microwave signal inside the filter. Therefore, changing the position of the cross-coupler can produce filters that more or less mismatched depending on cross-coupler coupling valve.
A typical cross-coupler constitutes an electrically conductive wire like member with a small plate being fixedly mounted at each end of the member. The member is then mounted across a vertical wall located in the filter that separates two of the non-sequential resonating cavities. The filter is covered by a removable cover. A technician whom has been instructed to produce a filter at a precise frequency, connects the filter to a piece of test equipment. If the coupling is not at the precise value, then the technician is to remove the cover, manually alter the position of one end or both ends of the wire type member cross-coupler, then replace the cover in position on the housing of the filter and then retest to determine if the coupling value is correct. If it is not the desired specific value, then the adjustment procedure is performed again and continues until the desired coupling is obtained. At times, it can literally take hours for a filter to be tuned to the precise coupling value because of the time involved in removing of the cover and reinstalling same.
The first embodiment of the present invention is to construct a tunable, cross-coupled bandpass filter which is formed of an enclosing housing which has a plurality of sequentially located resonator cavities. An input port is connected to a beginning cavity and an outlet port is connected to an ending cavity. A resonator is mounted within each of the resonator cavities. Each of the resonator cavities have an in-line coupler for coupling the electromagnetic signal between each sequential pair of resonators. A cross-coupler is disposed between a pair of non-sequential cavities. The cross-coupler includes a printed circuit (PC) board.
A further embodiment of the present invention is where the first basic embodiment is modified by the cavities being divided into a pair of side-by-side rows.
A further embodiment of the present invention is where the first basic embodiment is modified by there being located a vertical wall between at least two in number of the cavities that are not in direct sequence.
A further embodiment of the present invention is where the first basic embodiment is modified by each of the cavities being of a square shape in transverse cross-section.
A further embodiment of the present invention is where the first basic embodiment is modified by each resonator being cylindrical.
A further embodiment of the present invention is where the first basic embodiment is modified by the PC board including a dielectric compression board.
A further embodiment of the present invention is where the first basic embodiment is modified by the PC board being formed of a dielectric layer and an electrically conductive layer.
A further embodiment of the present invention is where the first basic embodiment is modified by the PC board including at least one tuning screw passing through a hole in the PC board.
However, it is important that the copper layer 74 form edge layers at each longitudinal end of the fiberglass layer 72 such as edge layer 75. Edge layer 75 will alter the inductance of the magnetic field passing through the filter 10 by the close proximity of each edge layer 75 to a resonator 28. Each edge layer 75 covers the edge of fiberglass layer 72 but not the edge of the compression board 66.
A further embodiment of the present invention is where the first basic embodiment is modified by a cover being mounted on the housing of the filter with the cover being removable.
A second basic embodiment of the present invention comprises a cross-coupled bandpass filter for a microwave electromagnetic signal which takes the form of an enclosing housing that has a plurality of resonator cavities located in a sequential arrangement. Directly between each pair of cavities in sequence there is located an in-line coupler. A resonator is located within each of the cavities. A cross-coupler is disposed between a pair of the cavities that are not in sequence with a first portion of the cross-coupler being located within one cavity and a second portion of the cross-coupler being located within another cavity. A cross-coupler is mounted between those cavities with the cross-coupler including a tuning screw that is manually turnable relative to the cross-coupler.
A further embodiment of the present invention is where the second basic embodiment is modified by the cavities being located in a pair of side-by-side rows.
A further embodiment of the present invention is where the second basic embodiment is modified by there being a vertical wall located between a pair of cavities which are not in direct sequence.
A further embodiment of the present invention is where the second basic embodiment is modified by the resonator cavities each being formed square in transverse cross-section.
A further embodiment of the present invention is where the second basic embodiment is modified by each resonator that is mounted within each cavity being cylindrical.
A further embodiment of the present invention is where the second basic embodiment is modified by the cross-coupler including a PC board which is formed by a dielectric layer and an electrically conductive layer.
A further embodiment of the present invention is where the second basic embodiment is modified by the tuning screw being mounted in conjunction with the PC board.
A further embodiment of the present invention is where the second basic embodiment is modified by there being a pair of tuning screws mounted in conjunction with the PC board with these tuning screws being located in a spaced apart arrangement.
A further embodiment of the present invention is where the second basic embodiment is modified by there being mounted a removable cover in conjunction with the housing with the tuning screws protruding exteriorly of the cover.
A further embodiment of the present invention is where the second basic embodiment is modified by the cover being spaced from both the electrically conductive layer and the resonators.